1. Field of the Invention
The invention relates to a precipitation hardening nickel alloy having a 0.2% proof stress of at least 500 N/mm.sup.2 and very good resistance to corrosion, the invention also relating to the use of said alloy for the making of structural components required to meet the aforementioned demands and to a process for the production of such structural components.
Very high resistance to corrosion means that the alloy and components made thereof can be exposed at temperatures between room temperature and 350.degree. C. and pressures between 10 and 100 bar to solutions containing CO.sub.2, H.sub.2 S, chlorides and free sulfur.
Such conditions are typically found in oil and natural gas exploration and production. Structural components meeting the aforementioned conditions have hitherto been made from nickel-based materials alloyed with chromium and molybdenum, although their 0.2% proof stress is only approximately 310 to 345 N/mm.sup.2. Their strength can be enhanced by cold working, although at the same time a reduction in ductility must be tolerated. Moreover, as a rule strain hardening cannot be used with very large cross-sections, so that in such cases precipitation hardening materials must be resorted to. However, in highly aggressive sour gas conditions materials which can be given higher strengths by precipitation hardening have inadequate resistance to corrosion, or they contain niobium as an essential alloying element required for precipitation hardening.
2. Description of the Prior Art
For example, J. A. Harris, T. F. Lemke, D. F. Smith and R. H. Moeller proposed a precipitation hardening nickel-based material containing 42% nickel, 21% chromium, 3% molybdenum, 2.2% copper, 2.1% titanium, 0.3% aluminium, 0.02% carbon, residue iron, which was alleged to be resistant in sour gas conditions (The Development of a Corrosion Resistant Alloy for Sour Gas Service, CORROSION 84, Paper No. 216, National Association of Corrosion Engineers, Houstin, Tex., 1984). However, their published results show that in conditions of extreme corrosion, such as may exist at greater depths, the material proposed is destroyed by stress corrosion cracking.
Another alloy was proposed in European Patent Specification 0066361. That proposed alloy contained (in % by weigh) in addition to 45 to 55% nickel, 15 to 22% chromium, 6 to 9% molybdenum, 2.5 to 5.5% niobium, 1 to 2% titanium, up to 1% aluminium, up to 0.35% carbon and 10 to 28% iron and other accompanying elements, also niobium as an alloying component essential for precipitation hardening. However, niobium-containing alloys are much less suitable for large scale industrial manufacture and processing than niobium-free alloys, since niobium-containing scrap and production wastes require a vacuum induction furnace for remelting if appreciable losses of this expensive alloying element by burn-off are to be avoided. Moreover, higher niobium contents, such as those here proposed, very clearly reduce the possibilities of hot shaping of the material. Similar disadvantages also apply to the alloy proposed by R. B. Frank and T. A. DeBold which have (in % by weight) 59 to 63% nickel, 19 to 22% chromium, 7 to 9.5% molybdenum, 2.75 to 4% niobium, 1 to 1.6% titanium, maximum 0.35% aluminium, maximum 0.03% carbon, residue iron (Properties of an Age-Hardenable, Corrosion-Resistant, Nickel-Base Alloy, CORROSION, 88 Paper No. 75, National Association of Corrosion Engineers, Houston, Tex., 1988). Due to its high nickel content, this alloy can also be expected to have a marked tendency towards hydrogen embrittlement in sour gas conditions in the temperature range below approximately 100.degree. C., so that in this respect it has limited utilizability.
The problem therefore exists of providing a precipitation hardening material which meets all the aforementioned requirements--i.e., has the required strength values, adequate resistance to corrosion in highly aggressive sour gas conditions, and requires no niobium for precipitation hardening.